In San Francisco, dozens of demonstrators, many of them old enough to have once protested against the Vietnam war in the 60s and early 70s, conducted a "die in" by lying on the sidewalk and pretending to be dead.

Some wore fake blood to recall the more than 3,200 US soldiers killed in the Iraq war.

Many later moved to obstruct Market Street, running through the city's central business district.

Neville Gittens, a police spokesman, said: "As soon as they went out there we started making arrests. They were warned."

Another spokesman said police arrested 57 people in two separate San Francisco locations.

Polls show most Americans now oppose the war in Iraq, and thousands have rallied against the war in recent days nationwide.